1. Field of the Invention
The present invention relates to a display device and to a method of driving the display device. Specifically, the present invention relates to a display device in which frame periods are structured by a plurality of subframe periods, the display device having a method of controlling the brightness of light emission by using the subframe periods as one of methods of controlling gray scales. The present invention also relates to a method of driving the display device.
2. Description of the Related Art
Along with the arrival of computerized industrial society, the demand for thin, flat-panel displays has increased recently, and the development of display devices using organic light emitting elements (hereinafter referred to as organic light emitting displays) has flourished. Organic light emitting displays are of self light emitting type, and a back light is unnecessary. Therefore, they are easier to be made thin compared with liquid crystal display devices. It is expected that they will be used in mobile telephones, personal digital assistants (EDAs), and the like.
Organic light emitting elements, also referred to as organic light emitting diodes (OLEDs), are light emitting elements. Organic light emitting elements each have a structure in which an organic compound layer is sandwiched between a cathode layer and an anode layer, and light emission is performed at a brightness corresponding to the amount of electric current flowing in the organic compound layer.
There is a method for displaying gray scales on an active matrix organic light emitting display referred to as an analog gray scale method. However, for cases of controlling gray scales by analog gray scale drive the amount of drain current changes greatly due to dispersion in the electric field effect mobility of driver TFTs formed as connected to organic light emitting elements, making the display of an image having uniform brightness difficult.
Drive by digital gray scales has thus been proposed as a means of achieving display having a uniform brightness. The term “digital gray scales” refers to a method of controlling gray scales by combining periods of light emission from organic light emitting elements with periods of no light emission.
A method referred to time gray scale drive exists as one of methods of driving by digital gray scales. The term “time division gray scales” refers to a method of performing gray scale display by dividing one frame period into a plurality of subframe periods, and controlling the emission of light or the non-emission of light by organic light emitting elements during each of the subframe periods.
However, it is known that false contours are generated, and image quality deteriorates, for cases of performing display by time gray scales. False contouring is a phenomenon in which unnatural light and dark lines be seen as mixed in an image when displaying half tones. (Nikkei Electronics, No. 753, pp. 152-62, October 1999; and “Pseudo Contouring Noise Seen in Pulse Width Fluctuation Dynamic Display,” TV Society Technical Bulletin, Vol. 19, No. 2, IDY9521, pp. 61-66.)
A method of separating and dividing the subframes of the longer time and higher order bits, for example, has been proposed as a method of preventing false contouring (JP 09-34399 A, JP 09-172589 A).
As stated above, problems develop with conventional time gray scale drive in that display disturbances due to false contours are generated, and display performance drops.
In order to control display disturbances caused by false contours with a conventional driving method, the subframe periods are separated and divided, for example, as discussed in JP 09-34399 A and JP 09-172589 A. However, if the false contours are prevented by the method of separating and dividing the subframe periods, a problem develops in that the electric power consumption increases.
That is, if the number of subframe period divisions increases, then the number of times that signals are input during one frame period increases. If the number of signal inputs increases, then the number of times that the electric charge is charged or discharged for giving the signals a desired electric potential also increases, and therefore the electric power consumption increases. In addition, if the number of divisions of the subframe period increases, then it is necessary to drive a driver circuit at a high frequency in order to fit the divided subframe periods into one frame period. The driving voltage becomes higher with high frequency drive, and therefore the electric power consumption, determined in proportion to the product of the driver frequency and the square of the driving voltage, increases.
In addition, there are cases with which it is not possible to apply the above method of dividing the higher order bit subframe periods with a driver circuit having low driver performance. This is because, even if an increase in the number of divisions of the subframe periods is attempted in order to reduce false contouring, there are cases in which the divided subframe periods cannot be fit within one frame period with the low driver performance driver circuit, and a limit on the number of divisions of the subframe period thus develops.